Pitot-static tubes, which are also called Prandtl tubes, are used on aircraft as speedometers. The actual tube on the aircraft can be around ten inches long with a half inch diameter. Several small holes are drilled around the outside of the tube and a center hole is drilled down the axis of the tube. The outside holes are connected to one side of a pressure transducer. The center hole in the tube is kept separate from the outside holes and is connected to the other side of the transducer. The transducer measures the difference in pressure in the two groups of tubes by measuring the strain in a thin element using an electronic strain gauge. The pitot-static tube is mounted on the aircraft, or in a wind tunnel, so that the center tube is always pointed in the direction of the flow and the outside holes are perpendicular to the center tube. On some airplanes the pitot-static tube is put on a longer boom sticking out of the nose of the plane or the wing.
Since the outside holes are perpendicular to the direction of flow, these tubes are pressurized by the local random component of the air velocity, or static pressure. The center tube, however, is pointed in the direction of travel and is pressurized by both the static pressure and the ordered air velocity (dynamic pressure). The pressure in the pitot-static tube is the total pressure and, by measuring the difference in total and static pressure, the pressure transducer calculates the dynamic pressure. Bernoulli's equation states that the static pressure plus one half the density times the velocity squared is equal to the total pressure. Therefore it follows that velocity squared equals the product of two times the dynamic pressure (difference in total pressure and static pressure) divided by density. Bernoulli's equation can then be used to calculate velocity by utilizing the dynamic pressure and the local value of air density from pressure and temperature measurements.
Besides aircraft speedometers, pitot-static tubes are coupled with additional aircraft instruments which are responsive to the impact pressures at the dynamic port of the tube and the static pressures at the static ports. For example, the altimeter and vertical velocity indicator are responsive to static pressure conditions, while the air speed and Mach number indicator are responsive to both dynamic and static pressures. Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Occasionally these instruments, various associated components, or the lines interconnecting the pitot-static system and the ports of the pitot-static tube develop leaks, causing the instruments to give erroneous readings. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot-static system, such as altitude, is potentially safety-critical. Thus the entire system associated with the pitot-static tube must be tested frequently to insure that no leaks are present which would cause erroneous readings.
Pitot-static tube testing adapters are used to perform anemometric tests by inducing known pressures in the dynamic and static lines leading from the pitot-static tube of an aircraft. Such adapters utilize seals to facilitate leak-free connections to the dynamic port and static ports. The testing adapters must be simple and reliable in design to ensure easy and repeatable installation. Also, in order to maintain the integrity of the pitot-static tube, testing adapters must not damage the tube or ports.
Various types of pitot-static tube testing adapters are commercially available, dependent upon the pitot-static tube design of the aircraft. Some pitot-static tube testing adapters are retained on the aircraft skin, while others are attached only to the pitot-static tube. Ultimately, the pitot-static tube tester must fit the design of the aircraft in such a way that a leak-free seal is made with both the center hole and outside static holes of the pitot-static tube without damaging the aircraft or the pitot-static tube probe, including the addition of any undue stress loading or probe misalignment.
One particular commercially available pitot-static tube testing adapter is made to be compatible with the downward-extending blade-type pitot probes found on the Piper Aircraft line, including Archer, Arrow, Warrior, Seminole, and Seneca models. This adapter mounts directly to the pitot-static tube, utilizing a clamp to attach the tester to the pitot-static tube. A rubber seal is glued in place on both sides of the clamp, the bottom rubber seal to prevent damage to the body of the tube and the top rubber seal to promote a leak-proof connection to the pitot or dynamic ports. However, the top dynamic seal often leaks and removal of a worn or leaking seal requires cutting the seal out and then scraping the adhered remnants off with a blade, the difficulty of which is exacerbated by awkward access and space constraints.
Given that the foregoing pitot-static tube testing adapter suffers from one or more limitations relating to the pitot port connection between the testing adapter and pitot-static tube, there is a need to develop a new, economical, better-performing, and replaceable sealing solution for such a testing adapter.